In recent years, from a standpoint that a recycling society should be constructed, movement to reduce usage of chemical fertilizers and promote usage of organic fertilizers has become active worldwide.
However, in hydroponics which is expanding in production of vegetables such as tomatoes and flowering plants, addition of organic materials to a nutrient solution has not been realized because a direct addition of organic materials to the nutrient solution causes generation of harmful intermediate decomposition products which cause damage to the root of the plants. Therefore, only chemical fertilizers are used for a nutrient solution at present.
On the other hand, various attempts have been made to provide techniques to utilize organic materials even for hydroponics.
For example, a trial has been made to mineralize organic materials prior to utilization for a nutrient solution (Refer to Patent Documents 1 to 3 and Non-patent Document 1). However, decomposition of a nitrogen component did not proceed beyond the production of ammonia by this method, presumably because decomposing a large amount of organic materials at one time in order to increase efficiency all the more generated a large amount of intermediate decomposition products, such as low molecular weight organic compounds and ammonia, resulting in destroying the nitrifying bacteria which decompose ammonia into nitric acid. Therefore, effort to utilize ammonia for a nutrient solution has been made but limited to utilization as a part of the fertilizer, because excess ammonia causes damage to crops.
Consequently, a method to utilize a nitrification reaction tank in order to generate nitric acid, which is more suitable as a fertilizer, from ammonia was developed (Refer to Patent Documents 4 to 7 and Non-patent Document 1). However, this method requires two reaction tanks, i.e., one tank for ammonification in which organic nitrogen is decomposed to ammonium nitrogen and the other tank for nitrification in which ammonium nitrogen is oxidized to generate nitrate nitrogen. Furthermore, it was difficult to stably obtain nitric acid because the nitrification reaction was unstable. In addition, there was a problem of bad smell due to incomplete decomposition in many cases.
In another method, a porous solid medium inhabitable for nitrifying bacteria is utilized to allow direct addition of organic materials into the nutrient solution (Refer to Patent Document 8). However, this was a restrictive technique because it requires utilization of a specific solid medium and cannot apply to other culture technologies, such as rock wool culture and water culture, which are in widespread use at present.
Thus, a technique to directly add organic materials to a nutrient solution applicable to any hydroponics technologies has not been put into practical use yet.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-137979    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-010885    [Patent Document 3] JP 2000-264765 A    [Patent Document 4] JP 2005-074253 A    [Patent Document 5] JP 2004-099366 A    [Patent Document 6] JP 2003-094021 A    [Patent Document 7] JP H06-178995 A    [Patent Document 8] JP H06-125668 A    [Non-patent Document 1] “Management of Nutrients and solid Media in Drip Fertigation” Hakuyusha, p. 119-155 (2005)